When a machine tool performs a cutting operation, it is necessary for the workpiece to be moved relatively to the cutting tool. Inaccuracies in this relative motion results in inaccuracies in the machined workpiece. It is therefore desirable to check periodically that relative motion between parts of the machine tool take place with acceptable accuracy.
This can be done by mounting a ballbar between, for example, the workpiece supporting table and the tool holder of the machine tool, and then causing the tool holder and the table to perform a circular movement about an axis passing through one end of the ballbar.
A ballbar is a telescopic bar which incorporates a highly sensitive position sensor. Any variation in the length of the bar is detected by the position sensor to provide a signal which can be processed by appropriate software to show any discrepancy between the actual and desired relative motions between the tool holder and the table.
In a perfectly accurate machine, the circular relative movement between the tool holder and the table would take place without any variation in the length of the ballbar. However, in practice, many factors, such as lateral play, backlash and control errors will cause the relative motion to vary from the perfectly circular, and consequently there will be changes in the length of the ballbar during the test.
Most machine tools have a workpiece supporting table which is movable in two mutually perpendicular directions in its own plane. Furthermore, either the tool or the table is movable towards and away from the other. Conventionally, the tool axis is referred to as the Z axis, and the directions of the other two motions are referred to as the X and Y axes.
Currently available ballbars are supplied with mounting components for supporting the ends of the ballbar respectively on the table and the tool holder. The mounting components are adapted for use with CNC milling machines that have removable tool holders that can receive the respective mounting component. Furthermore, the mounting components available for use with ballbars are capable only of checking relative movements in the X Y plane, which, in a vertical machining centre, is the plane of the workpiece supporting table.
The currently available ballbar kits are consequently not suitable for use with horizontal grinding machines (i.e. grinding machines in which the rotational axis of the grinding wheel is horizontal). Such machines have grinding wheel arbors instead of tool holders. It is not possible to fit existing ballbar mounting components to such arbors. Furthermore, since motion of the grinding wheel towards or away from the workpiece (in the Y axis direction in a horizontal grinding machine) is of fundamental importance in grinding operations, it is desirable to be able to use a ballbar to check for inaccuracies in all three planes.